How do particle colliders work? What kinds of energies are we talking about? Are there any uses for colliders except for physics experiments? I discuss these questions and more in today’s Ask a Spaceman!

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EPISODE TRANSCRIPT (AUTO-GENERATED)

It's not often that I get to start out an episode by discussing a unit used in physics, which on the surface doesn't sound like the most exciting hook for an episode intro, but in this case I just can't help myself. We're all used to the usual units, meters, parsecs, seconds, joules, Kelvin, all those, but here's one you probably haven't heard of yet. The barn. The barn is a unit of area. It's equal to a square 10 femtometers on a side. And 1 femtometer is 10 to the minus 15 meters. That's a millionth of a billionth of a meter. So this is not a very large area at all. But during the Manhattan Project work on the development of the atom bomb, a couple physicists over at Purdue University were using early particle accelerators to measure the cross-sections of some nuclear reactions. As in, how close two nuclei had to be to get them to do interesting things with each other. and they kept encountering this area again and again, 10 femtometers on a side.

Once you got to nuclei to within an area of around 10 femtometers on a side, they started doing interesting things. Since this common area was cropping up so much in their calculations they wanted to simplify their lives and create a new unit so they didn't have to keep writing, you know, 10 square femtometers, they wanted to write something else. So they tossed around names like, should we call it the Oppenheimer, and that's a little too long, or the Bethe, which is a giant in nuclear physics but easily confused with the Greek letter beta. Eventually, Keep in mind, this is at Purdue University in the heart of the Midwest, one of them suggested the barn. As in, that thing is as big as a barn. Or, you couldn't hit the broad side of a barn. You know, a barn. And now it's a thing. And it's glorious.

One of the few times that I wholeheartedly approve of a physics or astronomy naming convention Probably because I'm from the Midwest myself and I will always have a soft spot in my heart for anything Midwestern, although that soft spot in my heart is right next to all the plaque buildup from cheese consumption. Aside from being cute, the barn does give us the right reference frame to think about particle colliders. To get two atomic nuclei or any other associated particles to collide, you generally have to get them to within a barn of each other, which as you might imagine is rather challenging to do. And if you want anything interesting to come out of the reaction, like unveiling the deepest secrets of nature, then you have to do this at incredibly high energies and speeds, which is even harder to accomplish.

Hence the particle accelerator, which is a device that sounds like it only belongs in the realm of physics experiments, but with the exception of the various highest energy ones, over 99% of the accelerators on the planet are used for all sorts of stuff. The simplest, let me show you what I'm talking about. The simplest particle accelerator that you may be familiar with are the old school CRT TVs and monitors. CRT stands for cathode ray tube. These aren't anything, basically, if it's not a flat screen, if you go and find an old TV and it's a big boxy thing, you're looking at a CRT TV or monitor and that CRT stands for cathode ray tube. Cathode rays are an old name for electrons. Before we realized that electrons were particles, we thought they were a new kind of electromagnetic wave called cathode rays. Turns out not to be the case, but in some applications, the name stuck. So a cathode ray tube is a particle accelerator.

You start with the source of electrons at the back, usually just a hot bit of metal that the electrons can fly off of. And then in between, in a big empty box, you have a way to accelerate them. And the best way to accelerate charged particles is with an electric field. You just push them with an electric field. And then you need some way with a CRT monitor to curve their paths, curve the path of these electrons and paint them onto a target. And the best way to curve the path of a charged particle is to use a magnetic field, because that's what magnetic fields do. They curve the paths of charged particles. And then at the other end, you have a screen that lights up when the electrons hit it. That's how you make a TV. They're not used in TVs nowadays, but there was a time when every single household had a small particle accelerator in their living room.

And the earliest forms of particle accelerators used in more serious applications than, you know, television shows were just souped up versions of this. If you've ever seen a Van de Graaff generator at a science museum, this is what's happening inside of Van de Graaff generators. There's a big fabric belt that spins around a rubber belt that spins really, really fast, and it builds up lots and lots of charges on an inside section and then it generates a huge potential difference and then you just let the electron shoot off and you watch what happens. You can make electric sparks, you can hold on to it, it'll make your hair stand on end. These are souped up versions of what's happening in a cathode ray tube television and they are particle accelerators in their own right. But these kinds of accelerators are extremely limited in how much energy they can produce. In other words, how much of a kick they can give a particle.

Because you're limited by the size of the chamber and how big of an electric field you can put in that chamber. That's it. It's like a ball rolling down the hill. Its maximum speed is limited to how steep the hill is and how far it has to go to the bottom. Once you reach the limit, you reach the limit. You can only, if you build a house-sized Van de Graaff generator, yeah, you'll get a big electric potential, you'll get really charged up particles, you'll get some pretty high speeds, but that's the limit. You can't go faster, you can't go to even higher energies.

So in the 1930s, physicists developed a more powerful version of a particle accelerator known as a linear particle accelerator or LINAC when you want to play it cool. The idea is to repeat this process of accelerating particles as many times as possible, giving it a boost with each repetition by stacking a bunch of mini-accelerators in a row.

Here's the idea. You take a cavity, an empty shell, and you set up an electric field inside of it. I don't care how you do it, maybe you put an electric current around the edges of the cavity, maybe you use a big magnet, whatever you want to do, you get a big electric field going on in that cavity. Now, the cool thing about electric fields and charged particles is that they can push or pull depending on the direction. An electric field pointing with a charge can push it in one direction, and if it's pointing in the opposite direction as the charge's motion, it can pull on a charge. It's like if we had anti-gravity in addition to normal gravity. Gravity always pulls, but imagine if we could flip a switch and make gravity push. We can't. But it would be like this if we could. So what you start out with in this cavity is an electric field pointing in one direction that pulls on the charge. So you got a charge entering one side of the cavity. You have an electric field pulling on it.

So it drags it in. And then once the charge reaches the midpoint, you flip the electric field over and make the charge push out the other end. So you pull it in and then you push it out. And then, you have another cavity after that, and another cavity after that, and after that, after that, and each one of these synchronize so that cavity one pulls in a particle, then pushes it out. Then the next cavity pulls in the particle, then pushes it out. Then the next cavity pulls in the particle, then pushes it out. And on and on the charge goes, it's like the charge is riding a wave. In fact, we use electromagnetic waves and our frequencies to modulate this. So we always get this cycling between pushing and pulling electric fields. It's like a surfer riding a wave. But then when the wave dies down, there's another wave right behind it that the surfer can catch. And then as soon as that wave dies, the surfer catches another wave and another wave and another wave.

We just put a whole bunch of these in a row and we can get Not to light speed, but we can get pretty dang close. The current largest operating linear collider is the Stanford Linear Accelerator outside of San Francisco, which is 2 miles or 3.2 kilometers long and it does what I just described a whole bunch of times. The Stanford Linear Accelerator, or SLAC, accelerates electrons to energies of 50 gigaelectronvolts. Now I know I've already done one physics jargon unit today, but I don't think I did any in the last episode so I have a freebie that I'm going to cash in today for a second one. And that's the electron volt. This is a unit commonly used in particle physics just like the barn. It's defined to be the energy required to accelerate one electron over a potential difference of one volt. It's like, how much energy do I need to bring a one kilogram mass up a one meter height difference on the surface of the earth? It's like that analogous kind of definition.

How much work do I need to put in if there's an electric field here and I'm taking a charge and I'm pushing a charge through an electric field? How much energy do I have to put in to take that one electron and put it move it over one volt of potential difference? Seems kind of weird, but hey, particle accelerators are all about accelerating charged particles like electrons over potential differences, so this comes in handy to give you some reference frame. 1 electron volt is 10 to the minus 19 joules, which is not a lot. At Slack, we've got 50 giga electron volts, which means we have 50 billion electron volts. 50 billion electron volts is about 10 to the minus 8 joules. That's about a billion times weaker than the kinetic energy of a thrown baseball. To give you some perspective, which doesn't sound like a lot, but remember we're talking about subatomic particles here.

So all that energy, even though it's not a lot of total energy, if you throw a baseball, you'll have more energy in that baseball than you will have in these electrons at Stanford's linear accelerator, but all that energy is crammed into a very, very tiny volume. Remember, we're hitting barns here at 10 to the minus 15 meters, so it's crammed down into a very, very small area which means all of this energy is deposited onto a very small target which means fun things happen. If you were to put your hand in the center of the beam it would not feel like a thrown baseball hitting your hand it would just punch a hole in your hand. That's why, say, rubber bullets are less lethal than metal bullets, because the rubber bullets are bigger, they're softer, they're cushier, so that when they hit you, the kinetic energy is distributed over a larger area.

Yeah, it will hurt, it will give you a nasty bruise, but if you pack that same amount of energy into a smaller volume, into a smaller package with a tiny little bullet, the bullet will do more than bruise. If I throw a baseball at you, Yeah, it might hurt a little, and I'm very sorry about that, if I take even a billionth of that energy, but package it down into a single electron, that electron is going to mean business because it is accelerated to nearly the speed of light, and that single electron has all that energy, rather than all the energy being distributed across the entire baseball, which contains 11 billion electrons. I don't know the precise number off the top of my head, I just know it's a lot. So once we have these souped up particle colliders that are capable of achieving massive amounts of energy and putting that energy in a very, very small package, so small that it could hit the broad side of a barn, what do you get with that?

Like I said, 99% of the particle accelerators in the world are not used for physics at all. You encounter particle accelerators all the time, even if you don't have an old school CRT TV anymore. And if you do, you should probably consider upgrading. Go to a dentist or doctor's office, you get an x-ray. How do you think those x-rays are produced? That's high energy radiation. There's a little mini particle accelerator in there that bombards a piece of metal with electrons. That metal releases energy in the form of x-rays. That's how we energize it to get the x-rays to come out. That's how an x-ray machine works. There's a little particle accelerator in there. If you've ever had for yourself or encountered someone, a loved one going through particle therapy, you accelerate particles with a particle accelerator, and then you shoot them at tumors to kill the tumors without destroying the surrounding tissue.

It's like a little microscopic laser beam, except it's not laser, it's particles, electrons, neutrons, protons, depending on the application. We shoot at your tumor with little neutron bullets. accelerated with a particle accelerator. Medical imaging, like PET scans, require all sorts of isotopes of various elements. And yeah, we can just dig those isotopes out of the ground, but sometimes they're really hard to come by. Sometimes it's much easier to produce them with a collider. You take neutrons, protons, electrons, you accelerate them to nearly the speed of light, then you shoot them at a target, it transforms the target. They can undergo nuclear reactions. You're like, oh man, look at that element, I wish I had a couple fewer neutrons, then it'd be really handy for my PET scans.

Well, if I shoot stuff at it, and that atomic nuclei is about the size of a barn, and I shoot protons at it at nearly the speed of light, bing, bing, it clicks off a couple of those neutrons, I've transformed the element into a different isotope, there I go. I'm ready to go with my PET scan materials. There are ion implanters, which is not as scary as it sounds. This is used to create microscopic transistors for semiconductors. So every semiconductor in every computer chip you use in your smartphone, in your laptop, in your earbuds, everything, there was a particle accelerator involved in the manufacturing process. Remember, we're operating at the scale of barns here. This is how we make super, super tiny electronic circuits. Engineers use particle accelerators all the time for non-destructive testing. You shoot x-rays or neutrons in a material to look for tiny cracks and flaws. without actually breaking the thing you're trying to look at. You're trying to look at it microscopically.

Particle accelerators are used in the development of plastics. You shoot high-energy electrons at polymers. This gets them to mix up in interesting ways and cross-link with each other. Makes high-strength, versatile polymers for cable insulation. Your tires were made with a particle accelerator, folks. Let that sink in. This is how ubiquitous particle accelerators are. Your tires were made with a particle accelerator. Power plants are spewing out sulfur dioxide, nitrogen oxides, all sorts of nasty stuff. Bombard them with your particle accelerator. Shoot stuff at them. Transform them. Get them to trigger chemical reactions that transform them into harmless substances. You got a big cargo coming into port. You want to know what's inside of it, but you want to do it quickly. It's a little bit cumbersome to open up everything and open up every box. Shoot x-rays at it. Shoot neutrons at them. See how the x-rays bounce off. See how the neutrons bounce off. figure out what's inside of them.

Check out their innards, their circuitry, their components, the bits of metal that are in there. Hey, you want some ultraviolet rays, some x-rays to study crystals or molecules, proteins, applications in chemistry, biology, material science, you need a source of UVs and x-rays? Particle accelerators are the way to do it. First you accelerate particles, super easy. got a linear collider, just do it, get that particle going to nearly the speed of light, then slam it at a metal, or some substance, some material, and the material starts glowing, and not in just any normal fashion starts glowing in x-rays and UVs, and then you use the UVs and x-rays for something else. And of course, you can use particle colliders for particle physics. In our biggest applications, our biggest explorations of the workings of the subatomic world, we need even more energies than linear colliders can provide. And the problem with linear colliders is eventually they end.

They have a starting point and an ending point. And once you've reached that last cavity, that's it. There's no more acceleration. And the best way to make a line be infinite is to turn that line into a circle. So if we bend our linear collider into a circle, that way the party never stops. We just keep ramping up speed. We go from one cavity to another, to another, to another. Then we end up back where we started. And hey, it's like a brand new cavity. So we just keep accelerating. This is where we get a litany of different kinds of accelerators with all sorts of cool names like Bevatron, Cyclotron, Synchrotrons, and you get this one for free, the Patreon. You knew that was coming. That's patreon.com slash P.M. Sutter where you can support the show and I greatly appreciate it. The biggest of these. Circular colliders, which are just linear colliders, but in a circle, are the synchrotrons.

We accelerate charged particles in cavity after cavity, and we use our good friend, stalwart, reliable, dependable friend, magnetic fields, to bend those particles into the path of a circle. We run into a challenge here when you just start accelerating little particles. You get really good at it, they get really close to the speed of light. 99% the speed of light, 99.9% the speed of light, 99.99% the speed of light and on. As the particle gets faster and faster, it's kinetic energy increases because it's going faster. But energy is mass, thanks to special relativity. E equals mc squared, doesn't matter what the E is, any kind of energy is a form of mass. So as the particle gets faster and faster, it gets heavier and heavier. And so if you just keep one fixed magnetic field, it will start circulating, that particle will keep its path bent, keep it within the circle, keep it within your particle collider, but then once it gets heavy, it starts to drift. I can't keep...

can't keep going down the line and it'll just hit a wall. So as the particle gets faster and faster and gets heavier and heavier, you need to increase the strength of the magnetic field in step with the growing mass and speed of the particle so that you can keep it on a straight path or circular path. So it has to be synchronous with the rotation and speed, hence a synchrotron. The largest of these is of course the Large Hadron Collider. Hadron is just a name for protons and related particles. The Large Hadron Collider features a ring 27 kilometers in circumference.

It holds 36,000 tons of magnets to create the magnetic field to bend the path of the particles and keep it in a circle but the amount of energy needed by these magnets the amount of current needed is so vast that it would just melt normal electrical like you can't just like run a wire to these kinds of magnets for the kind of power they're drawing because the wire will just melt so everything is a superconductor So there's no electrical resistance, there's no heat loss, which means this experiment is chilled to a temperature of negative 273.1 Celsius. That is colder than outer space, that's colder than the cosmic microwave background. It's actually, the particles are injected with a linear collider, you have a linear collider starting out to get Particles is like an on-ramp on the freeway, it gets particles up to speed, then they enter the circular collider. These particles, the protons, they reach a speed of 99.9997828% the speed of light.

nearly the speed of light it's not quite the speed of light can't quite do that not allowed to do that but pretty dang close the collider works in both directions at once so while the collider is operating one beam of protons is being sent in one direction and then right next to it there's another being sent in the opposite direction and everything synchronized and then at the last minute once they reach top speed where the magnetic fields can't keep up anymore this is the fastest they can go before the beam starts to drift they do one quick last minute alignment of the magnetic fields and send the beams colliding into each other the two proton beams intersect for a total collision energy of 14 tera electron volts or about a millionth of a joule Like I said, these beams are nasty. opened up the access port to the beam while it was active.

He did not lose a finger or get a hole in his hand, but it did trigger a whole bunch of safety alerts and he got a very, very stern talking to for violating procedure. These things are dangerous. Do not put your hand in the middle of the Large Hadron Collider beam. It will be nasty business because even though a millionth of a joule isn't even enough to warm up your coffee in the morning, when it's all crammed down to a single proton, a single beam of protons colliding with another beam of protons, this gets you more than any industrial application. This gets you more than particle therapy, non-destructive testing, sewage treatment. Did I mention sewage treatment? I think I forgot sewage treatment. You shoot electrons and protons and neutrons at sewage and it kills all the baddies leaving the water. But at these kinds of energies, we're not talking cargo screening, light sources, x-ray at the dentist's office. We're talking about access to the entire subatomic world.

Because when the Large Hadron Collider goes off and the beams collide, it's not necessarily about the energies. This is about energy densities. It's about how much energy you can pack into as small a volume as possible. And the energy densities achieved by the Large Hadron Collider have not been seen anywhere in the universe since the first second of the Big Bang itself. These are energies so extreme that forces of nature change, fundamentally change. Electromagnetism and weak nuclear forces go away. They're replaced by a unified force called the electroweak force. In that moment, in that collision, there are only three forces of nature. gravity, strong nuclear, and electroweak. There is so much energy released in these collisions, so much energy density, that new particles appear out of the vacuum. Why? Because energy is mass and mass is energy. You can transform energy into mass.

In the language of quantum field theory, at the location where these particle beams collide, we are injecting so much energy into all the quantum fields of nature that they all briefly get to create particles. Particles literally appear from the vacuum, the protons are destroyed, energy is released, that energy is transformed. into particles, most of which don't last long, exotic things. Various species of quarks, lots of neutrinos, composite particles like the pion and the kaon come spewing out, weird force carriers that can only exist in these kinds of conditions and then once everything cools off those force carriers disappear and we get our good old photons back and weak bosons. At the very highest energies in these particle colliders, we are recreating states of matter that the universe has not seen for billions of years and we are accessing some of the most fundamental physics possible. Of course, to do that, first you have to hit the broad side of a barn.

drop a review on your favorite podcasting platform, or just tell someone about the show. It really helps the show visibility. I truly do appreciate it. And keep sending questions. That is the number one way to keep this show going, is to keep sending me questions. AskASpaceman at gmail.com or askaspaceman.com for the website. But I can't go without thanking my top Patreon contributors this month. They are Justin G, Chris L, Alberto M, Duncan M, Corey D, Michael P, Nyla, Sam R, Joshua, Scott M, Rob H, Scott M, Louis M, John W, Alexis, Gilbert M, Rob W, Jessica M, Jules R, Jim L, David S, Scott R, Heather, Mike S, Pete H, Steve S, Lisa R, Kevin B, Michael B, Eileen G, Steven W, Deb A, Michael J, and Philibelle, thank you so much. And I will see you next time for more Complete Knowledge of Time and Space.

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